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 From the pages of
Agricultural
Research magazine
Turning the Phage on
Produce Pathogens
Even bacteria have their nemesis.
Tiny viruses, called phages, infect and kill bacteria naturally,
including the foodborne pathogens that sometimes make humans so
sick, they wish they were dead.
So why not put these phages to work on fresh-cut fruit,
thought ARS plant pathologists Britta Leverentz and William S.
Conway at the Produce Quality and Safety Laboratory in
Beltsville, Maryland.
Since phages home in on a bacterium's surface proteins, they
are very selective about their hosts. Phages specific for
Salmonella, for instance, would leave beneficial bacteria free
to multiply on fresh-cut produce and crowd out potential
pathogens, Leverentz explains.
What's more, these tiny viruses are natural, safe, and
ubiquitous. A small dropperful of fresh water from a stream or
lake, for example, contains an average 250 million phages.
Before antibiotics, phages were used to treat human infections
in the United States and are still used therapeutically in other
parts of the world.
Phages are already under study to control pathogens in
poultry, meat, and eggs. Leverentz and Conway are the first to
investigate their potential to reduce pathogens on fruits and
vegetables—both whole and fresh-cut. They are working under a
cooperative research and development agreement with Intralytix
in Baltimore, Maryland, which is providing known phages for
Salmonella Enteritidis. A patent application has been filed on
the use of phages with produce.
Human infections with S. Enteritidis usually come from products
containing eggs. But during the last 10 years, various
Salmonella strains, including S. Enteritidis serotypes, have
been associated with contaminated produce.
The peel or rind of intact fruits provides a physical and
chemical barrier to microbes, says Leverentz. However, the
growing market for fresh-cut produce could become another
channel for pathogens to reach humans.
Bacteria multiply rapidly on cut surfaces—especially if those
surfaces are not too acidic and have warmed up to room
temperature.
Leverentz chose fresh-cut melons, with low acidity, and
apples, with higher acidity, to test a cocktail of four
anti-Salmonella phages under conditions likely to occur during
normal food processing and storage. She inoculated each slice
with 1 million bacteria and 100 million phages.
The results on the melons were very encouraging, she says.
The phage cocktail consistently reduced Salmonella more than a
thousandfold on melon chunks stored at 40°F and 50°F and more
than a hundredfold on fruit stored at 68°F, or room temperature.
But on apples, the phages made no detectable dent in pathogen
numbers. Apparently the fruit's higher acidity reduced phage
survival, says Leverentz. But she and colleagues are already
looking for acid-tolerant phages or a way to buffer the viruses
against acid.
Conway says the industry is seeking alternatives to chlorine
and other sanitizers now in use. Bacteria are developing
resistance to chlorine, which can also be irritating to humans.
Solutions are often too dilute or too overused to reduce
bacteria more than 10- to 100-fold, he says. That's a far cry
from the industry's goal of 100,000-fold reductions.
Conway suspects the phages may have to be combined with other
treatments, such as natural bacteria-killing compounds, to reach
that goal for commercial use.—By Judy McBride, Agricultural
Research Service Information Staff.
This research is part of Food Safety, an ARS National Program
(#108) described on the World Wide Web at http://www.nps.ars.usda.gov.
Britta Leverentz and William S. Conway are with the USDA-ARS
Produce Quality and Safety Laboratory, 10300 Baltimore Blvd.,
Bldg. 002, Beltsville, MD 20705-2350; phone (301) 504-6128, fax
(301) 504-5107.
"Turning the Phage on Produce Pathogens" was published in the
July 2001 issue of Agricultural Research magazine.
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